Friedel Weinert instructs that in order to comprehend why physical, natural time, is different from human, social time, it must be appreciated that natural units of time pre-exist conventional units of time. Furthermore, Weinert notes how socially convened units of time are based on natural temporalities.

In order to grasp the distinction between physical and human time, it is important to distinguish natural and conventional units of time. Natural units of time are based on periodic processes in nature, which recur after a certain interval. They may be quite imprecise, like the periodic flooding of the Nile, on which the ancient Egyptians based their calendar year; or more regular, like celestial phenomena. Some basic units of time, like the day and the year, are based on natural units of time. For instance, the equatorial rotational period of the Earth is 23 h 56 min and 4.1 s; that of Uranus is 17 h (Zeilik 1988, 508). The tropical year—the time that the Earth needs for one revolution around the sun—has a length of 365, 242,199… days or 365 days, 5 h, 48 min and 46 s (see Moyer 1982; Clemence 1966). But the calendar year has 365 days and 366 in leap years, which gives the calendar year an average length of 365.2425 days. As calendar years cannot have fractional lengths, there will always be a discrepancy between the tropical and the calendar year. This difference led to the replacement of the Julian calendar by the Gregorian calendar (1582). The Gregorian calendar will remain accurate to within one solar day for some 2,417 years. One difficulty with the day and the year, as just defined, is that these units of time are not constant, due to slight irregularities in the motion of the Earth. Historically, this discrepancy has led to calendar reforms and redefinitions of the ‘second’ from a fraction of the rotational period of the Earth around the sun to atomic oscillations.

Whilst physical time is based on such natural units, human time is based on conventional units of time. The 7-day week, introduced by the Romans, the subdivision of the day into 24 h, of the hour into 60 min and of minutes into 60 s, the division of the year into 12 months and the lengths of the months into 30 or 31 days (except February), again introduced by the Romans, are all conventional units of time. They are conventional because they respond to human social needs about time reckoning although there may be no physical processes, to which they correspond. To give an example, the beginning of the year (1st January) is purely conventional, since there is no natural event, which would single out this particular date. Equally the beginning of the day at midnight is a convention. Note, however, that not all such conventions are arbitrary. The equinoxes, the summer and winter solstices correspond to particular positions of the Earth with respect to the sun. Already the Babylonians introduced the 7-day week and named the days of the week, like the Egyptians, according to the sun and the known planets: moon, Mars, Mercury, Jupiter, Venus and Saturn (Wendorff 1985, 118). The division of the year into 12 months (4000 B.C.) was inspired by the 12 orbits of the moon around the Earth in one tropical year. But this creates a problem of time reckoning because the time between lunar phases is only 29.5 Earth days (Zeilik 1988, 152; Wendorff 1985, 14), but the solar year has 12.368 lunar months. As a consequence, the length of the month is now purely conventional and no longer related to the lunar month. The division of the day into 2 9 12 h is explained by geometrical considerations. During the summer only 12 constellations can be seen in the night sky, which led to the 12 h division of day and night. According to the sexagesimal system, there are 10 h between sunrise and sunset, as indicated by a sundial, to which 2 h are added for morning and evening twilight (see Whitrow 1989, 28–29; Wendorff 1985, 14, 49). When the year and the day are set to start also depends on conventions and social needs. In ancient Egypt, for instance, the year began on July 19 (according to the Gregorian calendar), since this date marked the beginning of the flooding of the Nile (Wendorff 1985, 46). In the late Middle Ages there existed a wide variety of New Year’s days: Central Europe (December 25); France (March 21; changed to 1st January in 1567); British Isles, certain parts of Germany and France (March 25) (Wendorff 1985, 185; Elias 1988, 21f).

Despite these aspects of conventionality, it must be emphasized that the conventional units of time must keep track of natural units of time. For otherwise, conventional units of time will fall out of step with the periodicity of the natural units. The measurement of time is inseparably connected with the choice of certain inertial reference frames, like the ‘fixed’ stars, the solar system, and the expansion of galaxies or atomic vibrations (Clemence 1966, 406–409). It was one of the great discoveries of Greek philosophy to have realized that there exists a link between time and cosmology. The existence of conventional units of time thus presupposes the existence of natural units of time (Weinert 2013, 16-17).

Weinert, Friedel. 2013. The march of time: Evolving conceptions of time in the light of scientific discoveries. Berlin and Heidelberg: Springer-Verlag.